Not calculated when using Subs function

10 次查看（过去 30 天）
Deokjae Jeong
Deokjae Jeong 2016-9-15
评论： Deokjae Jeong 2016-9-17
When using Subs function,the output of v is (14*2^(2/5)*3^(3/5)*15^(3/5)*25^(2/5))/75, which is 8.7615770046893.
Why the output is not simply 8.761577004689 ?
syms px py w v a
v=(w*(a/px)^a*((1-a)/py)^(1-a))
a=0.4
px=3
py=5
w=70
v=subs(v)

请先登录，再回答此问题。

回答（2 个）

John D'Errico
John D'Errico 2016-9-15
Because that is not the exact number.
simplify(v)
ans =
(14*3^(1/5)*10^(2/5))/5
I'm sure you can see that as soon as you decide to write it in decimal form, no matter what, you cannot write it exactly. And since this is the symbolic TB you are talking about, what do you expect?
vpa(v)
ans =
8.7615770046893145084508459712354
  3 个评论
显示 1更早的评论
John D'Errico
John D'Errico 2016-9-16
vpa will give you a decimal (but still in symbolic form) approximation to the number, yes. So will double, with fewer digits as a double precision number.

请先登录，再进行评论。

Walter Roberson
Walter Roberson 2016-9-15
(14*2^(2/5)*3^(3/5)*15^(3/5)*25^(2/5))/75 is not 8.7615770046893 . The expression is only approximately 8.7615770046893 . A better approximation would be 8.761577004689314508450845971235390274526747228228561126229132624459900445322460260513472705827961752 and it can be shown that the result is irrational (and thus non-terminating). Another way of writing the expression is (14/75)*50^(2/5)*45^(3/5) which you can see involves fifth roots.
The purpose of the symbolic toolbox is to calculate exact solutions whenever possible. The decimal value you list is not an exact solution, only a decimal approximation of it.
If you want the decimal approximation, use vpa() or double()

类别

在 Help Center 和 File Exchange 中查找有关 Conversion Between Symbolic and Numeric 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by